Historically, transducer elements of ultrasonic imaging devices have employed piezoelectric transducers to receive and transmit acoustic signals at ultrasonic frequencies. The performance of piezoelectric transducers is limited by their narrow bandwidth and acoustic impedance mismatch to air, water, and tissue. In an attempt to overcome these limitations, current research and development has focused on the production of capacitive micromachined ultrasonic transducer (cMUT) elements. cMUT elements generally include at least a pair of electrodes separated by a uniform air or vacuum gap, with the upper electrode suspended on a flexible membrane. Impinging acoustic signals cause the membrane to deflect, resulting in capacitive changes between the electrodes, which produce electronic signals usable for ultrasonic imaging.
The nature of the signals produced by cMUT elements demands that they are located as close as possible to the electronic readout circuits, ideally on the same physical substrate. While there have been efforts to make cMUT elements compatible with complementary metal-oxide (CMOS) integrated circuits, the conventional approaches have relied on depositing and patterning layers to form cMUT structures after the CMOS process steps are complete. These approaches raise substantial financial and technical barriers due to the high cost of adding patterned layers to a finely-tuned CMOS process and due to the high process temperatures needed to deposit the high quality structural layers needed for micromachined devices. The production of a cMUT element using this approach may require temperatures higher than 500 degrees Celsius, at which point the metallization layers within the CMOS circuit elements may begin to form hillocks or to alloy with adjacent layers. These phenomena may render the integrated circuit non-functional or, at best, will severely reduce production yield. In short, the existing approaches have failed to viably integrate the ultrasonic functions of a cMUT into an integrated circuit.
Thus, there is a need in the art of ultrasonic imaging devices for a new and improved capacitive micromachined ultrasonic transducer. This invention provides a design and manufacturing method for such transducer device.